This invention relates in general to timing devices and more particularly to the manual preset type.
In cooking appliances such as micro-wave ovens wide variations in cooking time are required depending on the nature of the material being cooked. Some items require only a few minutes. Other items may need up to 30 minutes, requiring the timer to have a 30 minute range. A timer having even scale divisions is not desirable in an application of this type as the manual setting error is too large for short time settings.
One solution for obtaining accurate short timings in a timer having a long timing range is the two speed timer. In this type of timer, the cam shaft is driven at relatively high speed through the first part of its timing range to give short accurate timings. It is driven at low speed through the remainder of its timing range to provide the long timings. Prior to the present invention, this type of timer required a double gear train, one for high speed and one for low speed. It also required a gear shifting mechanism to change speeds at the proper point on the timer dial. This extra mechanism adds to the cost of the timer and also decreases the reliability as it adds to the sources of possible failure.
Another type of timer proposed for this application is the so-called "log" or non-linear scale timer. In this type of timer the cam shaft is driven by a pair of log or eccentric gears of constantly varying pitch radius. This arrangement provides a constantly varying cam shaft speed. This speed increases progressively as the timer runs from its maximum setting back to its OFF position. This gives relatively large dial divisions for the short timings which improves the setting accuracy. It also provides for the longer timings. This type of timer has the disadvantage of being confusing to the user due to the constantly varying scale divisions on the dial.